Consider: It is unclear if this visitor should be applied directly to Filter, or should be
walked accross the data structure by hand. The standard complient structure is well defined, and
this should negate the need for a formal visitor (we don't have internal structure we are
hiding).

There is still a very valid use for FilterVisitor, a instance may implement both FilterVisitor
and ExpressionVisitor and ExpressionVisitory in one direction, and a FilterVisitor and a
StyleVisitor in the other. The ability to directly focus on transforming data within a larger
structure is something a normal data walk can not accomplish in a scalable manner.

Method Detail

visitNullFilter

This is particularly used during data structure transofrmations, however the use of
null is not recommended. Please make use of Filter.NONE and Filter.ALL as placeholder
objects that communicate intent.